The present invention relates to a magnetic field variation measuring method and magnetic field variation compensating method for an MRI (magnetic resonance imaging) apparatus, and an MRI apparatus, and more particularly to a magnetic field variation measuring method for measuring a variation in a static magnetic field of an MRI apparatus, a magnetic field variation compensating method for compensating a variation in a static magnetic field of an MRI apparatus, and an MRI apparatus capable of implementing such methods.
The static magnetic field of an MRI apparatus should be constant, but when a metal mass (e.g., an automobile) moves near the MRI apparatus or the environment changes (e.g., the temperature changes), the static magnetic field may be undesirably varied.
In the past, measures for dealing with such variation in the static magnetic field have focused on restraining the cause of the variation, such as by magnetically shielding the MRI apparatus or air-conditioning the room where the MRI apparatus is installed.
However, a variation in the static magnetic field sometimes occurs even after the measures have been taken, and in this case, a problem arises in that the image quality is undesirably degraded.
It is therefore a first object of the present invention to provide a magnetic field variation measuring method that can measure a variation in the static magnetic field of an MRI apparatus.
Moreover, it is a second object of the present invention to provide a magnetic field variation compensating method that can compensate for a variation in a static magnetic field of an MRI apparatus.
Furthermore, it is a third object of the present invention to provide an MRI apparatus that can suitably implement the magnetic field variation measuring method and magnetic field variation compensating method.
In accordance with its first aspect, the present invention provides a magnetic field variation measuring method characterized in: placing I (wherein Ixe2x89xa71) RF probes, each of which has a combination of a small phantom capable of emitting an FID (free induction decay) signal and a small coil, in proximity of an imaging region of an MRI apparatus; transmitting RF pulses from said RF probes at a time when a reference magnetic field is to be measured, and receiving FID signals to determine reference frequencies fir (wherein i=1xe2x88x92I) from said FID signals; transmitting RF pulses from said RF probes at times when a magnetic field variation is to be measured, and receiving FID signals to determine frequencies fi from said FID signals; and determining a j-th order magnetic field variation xcex1j by solving the following equation:                     f        i            -              f        ir              =                  ∑                  j          =          0                          I          -          1                    ⁢                        α          j                ·                  r          i          j                      ,
wherein the position of each RF probe is represented by rj.
In the magnetic field variation measuring method of the first aspect, RF probes are placed and a reference static magnetic field is measured as a reference frequency in the beginning, and the static magnetic field is measured at appropriate times as a frequency, to determine the amount of a static magnetic field variation from their difference. If the RF probes are fixed, up to an (Ixe2x88x921)-th order magnetic field variation can, in general, be determined.
In accordance with its second aspect, the present invention provides the magnetic field variation measuring method of the aforementioned configuration, characterized in that I=2, and a zeroth-order magnetic field variation xcex10 and a first-order magnetic field variation xcex11 are determined.
In the magnetic field variation measuring method of the second aspect, the RF probes are fixed, and zeroth- and first-order magnetic field variations can be determined.
The zeroth-order magnetic field variation is a magnetic field variation that is independent of position, and the first-order magnetic field variation is a magnetic field variation that is a linear function of position.
In accordance with its third aspect, the present invention provides a magnetic field variation measuring method characterized in: placing two RF probes, each of which has a combination of a small phantom capable of emitting an FID signal and a small coil, across an imaging region of an MRI apparatus; transmitting RF pulses from said RF probes at a time when a reference magnetic field is to be measured, and receiving FID signals to determine reference frequencies f1r and f2r from said FID signals; transmitting RF pulses from said RF probes at times when a magnetic field variation is to be measured, and receiving FID signals to determine frequencies f1 and f2 from said FID signals; and determining a zeroth-order magnetic field variation xcex10 and a first-order magnetic field variation xcex11 by solving the following equations:             α      0        =                            (                                    f              1                        -                          f                              1                ⁢                r                                              )                +                  (                                    f              2                        -                          f                              2                ⁢                r                                              )                    2                  α      1        =                                        (                                          f                1                            -                              f                                  1                  ⁢                  r                                                      )                    -                      (                                          f                2                            -                              f                                  2                  ⁢                  r                                                      )                          2            .      
In the magnetic field variation measuring method of the third aspect, the RF probes are fixed, and zeroth- and first-order magnetic field variations can be determined.
In accordance with its fourth aspect, the present invention provides the magnetic field variation measuring method of the aforementioned configuration, characterized in that the time when the reference magnetic field is to be measured is immediately before the beginning of a pulse sequence for imaging a first view, and the times when the magnetic field variation is to be measured are immediately before the beginnings of pulse sequences for imaging second and later views.
In the magnetic field variation measuring method of the fourth aspect, when an imaging pulse sequence is repeated to collect data filling a k-space, a magnetic field variation is measured before the beginning of the imaging pulse sequence each time, and therefore a magnetic field variation when a metal mass moves near the MRI apparatus can be dealt with.
Particular examples of the imaging pulse sequence include pulse sequences that observe a gradient echoes, such as one according to GRASS (gradient recalled acquisition in the steady state) or SPGR (spoiled GRASS).
In accordance with its fifth aspect, the present invention provides the magnetic field variation measuring method of the aforementioned configuration, characterized in that the time when the reference magnetic field is to be measured is at startup of the MRI apparatus, and the times when the magnetic field variation is to be measured are at regular time intervals after the startup of the MRI apparatus.
In the magnetic field variation measuring method of the fifth aspect, since the magnetic field variation is measured at startup of the MRI apparatus and at regular time intervals after the startup, a magnetic field variation when the environment changes can be dealt with.
In accordance with its sixth aspect, the present invention provides the magnetic field variation measuring method of the aforementioned configuration, characterized in that the MRI apparatus is an open-type MRI apparatus that generates a static magnetic field in the vertical direction, and the RF probes are disposed above and below the imaging region.
In the magnetic field variation measuring method of the sixth aspect, a variation in the static magnetic field of an open-type MRI apparatus, in which homogeneity of the magnetic field is achieved by mechanical shimming or by adding a plurality of small pieces of magnet or iron, can be suitably measured.
In accordance with its seventh aspect, the present invention provides a magnetic field variation compensating method characterized in correcting the transmission frequency of an RF pulse and the receiving detection frequency of an NMR signal based on the zeroth-order magnetic field variation xcex10 measured by the magnetic field variation measuring method of the aforementioned configuration.
In the magnetic field variation compensating method of the seventh aspect, the zeroth-order static magnetic field variation can be compensated for by correction of the transmission frequency of an RF pulse and the receiving detection frequency of an NMR signal.
In accordance with its eighth aspect, the present invention provides a magnetic field variation compensating method characterized in correcting the gradient current based on the first and higher order magnetic field variations xcex1j measured by the magnetic field variation measuring method of the aforementioned configuration.
In the magnetic field variation compensating method of the eighth aspect, the first and higher order static magnetic field variations can be compensated for by correction of the gradient current.
In accordance with its ninth aspect, the present invention provides an MRI apparatus characterized in comprising: I (where Ixe2x89xa71) RF probes, each of which consists of a combination of a small phantom capable of emitting an FID signal and a small coil, disposed in proximity of an imaging region; reference frequency acquiring means for transmitting RF pulses from said RF probes at a time when a reference magnetic field is to be measured, and receiving FID signals to determine reference frequencies fir (wherein i=1 to I) from said FID signals; frequency acquiring means for transmitting RF pulses from said RF probes at times when a magnetic field variation is to be measured, and receiving FID signals to determine frequencies fi from said FID signals; and magnetic field variation calculating means for determining a j-th order magnetic field variation xcex1j by solving the following equation:                     f        i            -              f        ir              =                  ∑                  j          =          0                          I          -          1                    ⁢              j        ·                  r          i          j                      ,
wherein the position of each RF probe is represented by ri.
In the MRI apparatus of the ninth aspect, the magnetic field variation measuring method of the first aspect can be suitably implemented.
In accordance with its tenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that I=2, and a zeroth-order magnetic field variation xcex10 and a first-order magnetic field variation xcex11 are determined.
In the MRI apparatus of the tenth aspect, the magnetic field variation measuring method of the second aspect can be suitably implemented.
In accordance with its eleventh aspect, the present invention provides an MRI apparatus characterized in comprising: two RF probes, each of which consists of a combination of a small phantom capable of emitting an FID signal and a small coil, disposed across an imaging region; reference frequency acquiring means for transmitting RF pulses from said RF probes at a time when a reference magnetic field is to be measured, and receiving FID signals to determine reference frequencies f1r and f2r from said FID signals; frequency acquiring means for transmitting RF pulses from said RF probes at times when a magnetic field variation is to be measured, and receiving FID signals to determine frequencies f1 and f2 from said FID signals; and magnetic field variation calculating means for determining a zeroth-order magnetic field variation xcex10 and a first-order magnetic field variation xcex11 by solving the following equations:             α      0        =                            (                                    f              1                        -                          f                              1                ⁢                r                                              )                +                  (                                    f              2                        -                          f                              2                ⁢                r                                              )                    2                  α      1        =                                        (                                          f                1                            -                              f                                  1                  ⁢                  r                                                      )                    -                      (                                          f                2                            -                              f                                  2                  ⁢                  r                                                      )                          2            .      
In the MRI apparatus of the eleventh aspect, the magnetic field variation measuring method of the third aspect can be suitably implemented.
In accordance with its twelfth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that the time when the reference magnetic field is to be measured is immediately before the beginning of a pulse sequence for imaging a first view, and the times when the magnetic field variation is to be measured are immediately before the beginnings of pulse sequences for imaging second and later views.
In the MRI apparatus of the twelfth aspect, the magnetic field variation measuring method of the fourth aspect can be suitably implemented.
In accordance with its thirteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that the time when the reference magnetic field is to be measured is at startup of the MRI apparatus, and the times when the magnetic field variation is to be measured are at regular time intervals after the startup of the MRI apparatus.
In the MRI apparatus of the thirteenth aspect, the magnetic field variation measuring method of the fifth aspect can be suitably implemented.
In accordance with its fourteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that the MRI apparatus is an open-type MRI apparatus that generates a static magnetic field in the vertical direction, and the RF probes are disposed above and below the imaging region.
In the MRI apparatus of the fourteenth aspect, the magnetic field variation measuring method of the sixth aspect can be suitably implemented.
In accordance with its fifteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in comprising RF frequency correcting means for correcting the transmission frequency of an RF pulse and the receiving detection frequency of an NMR signal based on the measured zeroth-order magnetic field variation xcex10.
In the MRI apparatus of the fifteenth aspect, the magnetic field variation compensating method of the seventh aspect can be suitably implemented.
In accordance with its sixteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in comprising gradient current correcting means for correcting the gradient current based on the measured first and higher order magnetic field variations xcex1j.
In the MRI apparatus of the sixteenth aspect, the magnetic field variation compensating method of the eighth aspect can be suitably implemented.
According to the magnetic field variation measuring method for an MRI apparatus of the present invention, the amount of a variation in a static magnetic field caused by a metal mass (e.g., an automobile) moving near the MRI apparatus or an environment change (e.g., a change in temperature) etc. can be measured.
Moreover, according to the magnetic field variation compensating method for an MRI apparatus of the present invention, a variation in the static magnetic field of an MRI apparatus can be compensated for.
Furthermore, according to the MRI apparatus of the present invention, the magnetic field variation measuring method and magnetic field variation compensating method can be suitably implemented.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.